Assalamu Alaikum hopeonallah,
The first problem is the English language it does not have similar idioms and the richness of the ancient languages as is so with Arabic and even less with Fushat (the Arabic of the Quran). Mind you I don't speak it, but my mother tongue is as old as and has many similar saying and expressions (different language same expressions)
While you are thinking in one language it's hard to understand another so try and get into the spirit of the language and not the English equivalent words. Some explanations are as the brothers in previous post told you similar to English.
When you hear that memorising is from the chest you assume the heart "Qalb" where as the reference is to the "Fuad" the heart of the soul.
For when you die you body disintegrates along with your organs, but the spiritual self was there before you where born and still there after you're gone. The essence of you is in that "Fuad" pains, sorrows, sins, knowledge all.
The same applies for the brain its neurons, nerves and what have you, what it is not, it's not a recorder of your life that would be the "Fuad" again.
I and millions of others wish it was only the brain so when the worms ate it our bad self would vanish also but alas it ain't so.
So back to the verses such as "sealed their harts" would again reffer to unfeeling as the heart the emotional center which again could only be the "Fuad". The Quran was delivered to the prophet s.a.w's heart ?
You may ask why then does not Allah S.W.T just make it clear ?
There are of course many many theories but the best answer would be that He chose not to and He surely knows best.
You could to look at the Tafsir of those verses to get a analogy of different ways to accept the same.
Another point I would like to make that a lot of people say that this "Fuad" thing is from Sufis if that be true then what better explanation can they offer and mind you it would have to be in the realms of what would have been acceptable to those past generations as well. The truth does not change only perceptions do.
One last thing too, that the heart muscle as it may be and a pump it gets damaged from emotional stress does it not and it cant just be pumping issues otherwise athletes would die from stress; and a final neurological truth about the brain's true main function take a look at diagram of the body and see where the majority of these neurons end up. I'm sure then you will see the irony !
In today's world some of us place our hopes in science for they may find a solution that suits the hopes of our sinful self.
I hope I've not confused you more in my attempt to keep this brief,
Masalam
PS: The true mistery lies in how the one heart (physical) relates to the other (spiritual)
